AS 4254 Ductwork Construction Guide for Commercial Buildings
What You Need to Know
AS 4254 sets how ducts must be built, sealed, hung, and tested in Australia. Part 1 covers flexible duct. Part 2 covers rigid sheet metal duct. The NCC calls up both. If a duct fails leakage testing, hanger inspection, or fire material certification, the system does not get a compliance certificate.
This memo covers what installers need on site: gauge selection, sealing, supports, and the defects that get flagged at commissioning.
The Rules
- Rigid ductwork must be built and installed to AS 4254.2:2012, Section 2 (NCC 2022 J6D7)
- Flexible duct must comply with AS 4254.1:2021. It is rated for 1000 Pa positive and 200 Pa negative design pressure (AS 4254.1, Section 2)
- Duct systems of 3000 L/s or more must pass leakage testing with a maximum of 5% leakage at 1.25 times design operating pressure (AS 4254.2, Cl 2.2.4)
- Sheet metal gauges must follow the duct construction tables in AS 4254.2 Section 2.3, sized by duct width and pressure class
- Hangers and supports for rigid duct must comply with AS 4254.2 Cl 2.6.1 (straps and rods) or Cl 2.6.2 (wire rope) and be marked with manufacturer and Safe Working Load
- Duct materials must meet AS 1530.3 fire hazard properties: Smoke Developed Index of 3 or less and Spread of Flame Index of 0 (AS 4254.2, Section 1.8)
- Sealants, mastics, and gaskets used on joints must comply with AS 4254.2 Section 2.4 and meet the same fire properties as the duct itself
What This Means in Practice
AS 4254.2 sorts duct construction by pressure class. Most commercial supply and return systems sit in the low-pressure class. High-static systems and long mains run to higher pressure classes. The pressure class drives the sheet thickness, the seam type, and how often the duct needs stiffeners. A bigger duct at higher pressure needs a thicker gauge and closer reinforcement. The duct fabricator picks these from the AS 4254.2 Section 2 tables based on the duct width and the design static pressure.
Sealing class drives the leakage test result. AS 4254.2 does not use the SMACNA A, B, C labels directly, but the sealing principle is the same. Spec writers often borrow the SMACNA naming when calling up sealing scope: Class A seals all transverse joints, longitudinal seams, and penetrations. Class B seals transverse joints and longitudinal seams. Class C seals transverse joints only. For systems at or above 3000 L/s, AS 4254.2 effectively requires the equivalent of Class A on every joint and seam. Pick the wrong sealing scope and the duct fails the 5% leakage test at 1.25 x design pressure.
Hangers fail more often than the duct itself. AS 4254.2 Section 2.6 sets the support spacing by duct size, weight, and orientation. Wire rope hangers must be a matched, load-tested system with the manufacturer name and SWL stamped on each locking device. Site teams that mix-and-match wire rope with off-the-shelf grippers fail the inspection. Strap and rod hangers must be sized for the duct weight plus the insulation, not just the empty duct.
Flexible duct is the most-fixed item at commissioning. AS 4254.1, called up by the NCC, caps each flexible duct run at 6 m, single length, no intermediate joints. Site teams routinely run longer, kink at the connection, or leave 1 to 2 m of slack coiled in the ceiling. Compressed and bent flex duct can multiply the pressure drop several times over and wreck the airflow balance.
Key Design Decisions
Pressure Class Selection
Pick the lowest pressure class the system fan curve allows. A lower pressure class lets the fabricator use thinner gauges and wider hanger spacing. A higher pressure class suits compact mains in tight risers but adds gauge, stiffener, and sealing cost.
Sealing Scope
Specify the equivalent of SMACNA Class A sealing for systems at or above 3000 L/s. For smaller systems, Class C (transverse joints only) is the practical minimum.
Hanger System Type
Use strap and rod hangers for general office and retail. Use load-rated wire rope systems for plant rooms, exposed ducts, and renovation work where speed matters.
Flexible Duct Strategy
Limit flex duct to the final run-out from the rigid take-off to the diffuser. Cap single runs at 6 m, fully extended.
Who Needs to Know What
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References
- AS 4254.1:2021, Ductwork for air-handling systems in buildings, Part 1: Flexible duct
- AS 4254.2:2012, Ductwork for air-handling systems in buildings, Part 2: Rigid duct
- AS 1530.3, Methods for fire tests on building materials, components and structures, Part 3: Simultaneous determination of ignitability, flame propagation, heat release and smoke release
- AS/NZS 4859.1, Thermal insulation materials for buildings, Part 1: General criteria and technical provisions
- National Construction Code 2022, Volume One, Part J6 (J6D6, J6D7)
- AIRAH DA03, Ductwork for Air Conditioning
- SMACNA, HVAC Duct Construction Standards, Metal and Flexible (international reference for sealing class naming)